The present invention relates to a metal hub for an energy storage rotor. More precisely, the invention relates to a stiff metal hub for a flywheel that maintains tight interference fit with a radially-deflecting composite rim during high-speed operation by deflecting at the hub outer rim, which minimizes vibrations, and produces a critical velocity substantially higher than the design operating velocity.
Energy storage rotors, or flywheels, which internally produce and store kinetic energy, have been available as an alternative to batteries and other means of storing energy for about 30 years. Initially, flywheel assemblies were made of metal, e.g., high strength steel. However, flywheels made from composite materials provide superior energy storage capability to steel flywheels as the high-strength, lighter-weight composite flywheels can be rotated at greater speed. Recognizing that energy storage is proportional to the flywheel mass and the square of rotational velocity, substantially increasing rotational velocity and marginally decreasing mass by replacing steel with a composite material provides greater energy storage. Technological advances, thus, have made flywheel assemblies lighter in weight and capable of operating at higher operating speeds by using fiber composite materials, e.g., fiberglass or carbon fibers wound with a resin binder (carbon-carbon composite) in flywheel assemblies. The low density, high strength composite materials are ideally suited for flywheel assemblies, especially flywheel rings, which play a dominant role in overall flywheel energy storage.
High rotational operating velocities, however, produce extremely high centrifugal forces, which produce high radial and hoop stresses in the outermost composite rim. High stresses in the composite rim cause the rim to xe2x80x9cgrowxe2x80x9d radially, i.e., to deform outwardly in a radial direction. The flywheel hub, which holds the composite rim on a rotary shaft, generally by tight interference fit, is traditionally made of a high strength, lightweight metal alloy or a composite material. Metal alloy hubs provide strength and stiffness to the flywheel assembly. However, metal hubs often do not experience radial growth commensurate with, or of the same magnitude as, the deforming composite rim. Consequently, the composite rim separates from the hub, which produces potentially deleterious vibrations. Hubs made from composite materials as a rule are more flexible, which substantially minimizes separation between the hub and the composite rim. However, composite hubs as a rule are not sufficiently stiff to produce a critical velocity that exceeds design operating speeds.
As a composite rim separates from a hub, holidays, or gaps, in the tight interference fit appear between the hub and the composite rim, causing undesirable and potentially deleterious vibrations. Such vibrations are detrimental to the operation of the flywheel assembly. Moreover, if they occur at the natural, or resonant, frequency of the flywheel assembly and/or the component parts of the flywheel assembly, these vibrations could seriously damage or altogether destroy the flywheel assembly. Thus, those of ordinary skill in the pertinent art have focused a great deal of attention on means of solving the compatibility problem associated with flywheel assemblies having lightweight, high strength composite rims.
Medlicott (U.S. Pat. No. 4,821,599) discloses an energy storage flywheel with at least one (but preferably two or more) xe2x80x9csubstantially circular dished memberxe2x80x9d having an elastic modulus less than the elastic modulus of the composite ring. As the Medlicott flywheel rotates at higher velocities, the dished member deforms elastically causing the periphery of the dished member to move outwardly radially, maintaining contact with the less elastic composite ring, which also is expanding radially. However, with dished member hubs, having a lower modulus than the composite ring, the flywheel is less rigid and, consequently, more prone to vibrate than a system with a more rigid hub. Moreover, design operating speeds typically are greater than critical velocities, requiring the flywheel system during operation to transition through the critical frequency.
Flanagan et al. (U.S. Pat. No. 4,860,611) also discloses an energy storage rotor with a flexible rim hub. The Flanagan invention provides an expanding aluminum hub design on which a composite ring is shrunk-fit. The Flanagan hub includes a plurality of spokes that are joined at the periphery by a continuous rim. The sections of the rim between adjacent spokes are purposely made thinner, thereby allowing the rim sections between adjacent spokes to flex outwardly to maintain interference fit with the composite ring. Flanagan. et al. discloses that tight interference fit at lower operating frequencies and rim flexibility at higher operating frequencies substantially minimize vibrations and separation. Indeed, Flanagan expresses that critical frequency, i.e., resonance, does not occur because the rotor reaches its design operating speed, which is well above critical velocity, rapidly xe2x80x9cso that the rotor does not pass through potentially destructive critical frequency.xe2x80x9d However, in fact, the Flanagan flywheel system requires that the rotor pass through the critical frequency, subjecting the rotor to potentially deleterious vibrations.
Bitterly et al. (U.S. Pat. No. 5,124,605) discloses a flywheel with a xe2x80x9cself-restoring bearing systemxe2x80x9d that comprises a hub that is joined to a composite ring by a plurality of tube assemblies. The tube assemblies, which are attached to the hub and the composite ring, allow differential radial expansion in the hub and the composite ring. Hence, radial expansion of the hub does not have to be compatible with radial expansion of the composite ring. Indeed, the hub, which has a lower modulus of elasticity and, further, is purposely designed to expand more rapidly than the composite ring, compresses the plurality of pre-loaded tube assemblies, which absorbs the stress like a spring. The tube assemblies also maintain contact with the radially deforming composite ring. As before, a disadvantage of this hub is that the critical velocity is less than the design operating velocity, which may subject the rotor to potentially deleterious vibrations.
Swett et al. (U.S. Pat. No. 5,732,603) discloses a flywheel rotor with an expansion-matched, self-balancing, fiber or matrix composite hub, which includes an annular hoop and a pair of compliant diaphragms that are connected by the hoop. The diaphragms include an annular spring, the compliance of which facilitates maintaining contact at the hub-ring interface. Once again, a disadvantage of this hub is that the critical velocity is less than the design operating velocity, which may subject the rotor to potentially deleterious vibrations.
Swett (U.S. Pat. No. 6,014,911) discloses a flywheel rotor with a self-expanding hub having a double cone configuration. As the flywheel rotates, the hub surfaces flex, creating compression at the rotor that prevents matrix material of the rotor from pulling apart at high velocities. Here again, a disadvantage of this hub is that the critical velocity is less than the design operating velocity, which may subject the rotor to potentially deleterious vibrations.
Waagepetersen (U.S. Pat. No. 5,946,979) discloses a flywheel with an expansive, thin-walled, conical or fructo-conical hub, which expansion is made possible by either attaching a flexible, fiber-reinforced plastic material at the end of the hub or adhesively connecting the hub directly to the ring. Here again, a disadvantage of this hub is that the critical velocity is less than the design operating velocity, which may subject the rotor to potentially deleterious vibrations.
Fullwood et al. (PCT WO 97/1313) discloses a conical, fiber-reinforced end cap (xe2x80x9chubxe2x80x9d) that deflects towards a planar configuration as the rotor speed increases. Such deflection ensures that the rotor and end cap maintain their initial strain. Here again, a disadvantage of this hub is that the critical velocity is less than the design operating velocity, which may subject the rotor to potentially deleterious vibrations.
Thus, it would be desirable to produce a flywheel hub for attaching a low-density, high-strength, high-growth composite rim to a rotating, high-strength, metal flywheel shaft in such a manner as to substantially maximize the energy storage capacity of the flywheel assembly; to substantially minimize loss of interference fit between the hub and composite rim at very high rotational speeds; and to substantially minimize potentially destructive or deleterious vibrations that may result therefrom.
Therefore, it is an object of the present invention to provide a stiff metal hub for connecting a high-strength metal flywheel shaft to a low-density, high-strength composite rim.
It is a further object of the present invention to provide a stiff metal hub that maintains interference fit with the composite rim at very high rotational velocities, minimizing separation.
It is another object of the present invention to provide a stiff metal hub that substantially minimizes vibrations during high speed operation of the flywheel.
It is a yet another object of the present invention to provide a stiff metal hub that produces a critical speed substantially greater than the design operating speed of the flywheel.